A thermal fuse has widely been used in a variety of electric home appliances, mobile equipment, communication equipment, business equipment, vehicle-mounted equipment, AC adapters, chargers, motors, batteries and other electronic components as a protective component accurately detecting abnormal overheating of the equipment to rapidly interrupt a circuit or allow the circuit to conduct. Conventionally, thermal fuses have been categorized mainly in two types depending on the fuse element or thermosensitive material used: a thermal fuse using conductive, low-melting fusible alloy; and a thermal fuse using non-conductive, thermosensitive material. These fuses are both a so-called non-reset thermal switch operating in response to an abnormally increasing ambient temperature to interrupt equipment's current or provide a current path with a conducting state to protect the equipment. It operates at a temperature determined by the thermosensitive material used. Typically, it is offered in products as a protective component functioning at a temperature ranging from 60° C. to 250° C. on a rated current ranging from 0.5A to 15A and acts as an electrical protection means allowing an initial conducting or interrupt state for ordinary temperature to be inverted at a predetermined operating temperature to provide an interrupt or conducting state.
The thermal fuse using non-conductive thermosensitive material is typically configured as follows: A cylindrical enclosure has opposite ends each with a lead attached thereto and an organic chemical agent having a prescribed melting point is molded into a predetermined geometry to obtain a thermosensitive material which is then accommodated in the enclosure and for which a compression spring or the like exerts force on a movable conductor to configure the fuse. For example, Japanese Patent Laying-Open No. 10-177833 describes a thermal fuse having an enclosure in the form of a glass tube which has an internal portion provided with a pair of conductive films and accommodates successively a thermosensitive material, a conductor movable between a conducting position and an interrupt position, and a compression spring exerting force on the movable conductor with an insulator posed therebetween.
The thermal fuses using thermosensitive material as described above employ a relatively pure organic chemical for the thermosensitive material. More specifically, this substance is granulated and molded into a predetermined form to provide the thermosensitive material. It is, however, susceptible to the material's softening, deformation, sublimation, deliquescent property and other surrounding, environmental conditions and there have been a large number of concerns in terms of management of production steps, conditions for storing the finished product, and the like. For example, Japanese Patent Laying-Open No. 2-281525 describes that a residual stress introduced when a casing accommodating thermosensitive material and an external leading lead are crimped and thus fixed introduces a gap, which allows external moisture to enter the casing and negatively affect the thermosensitive material. When thermosensitive material having deliquescent property is exposed to external air, the material deforms, sublimates and the like. Accordingly in molding such thermosensitive material a complete management of sealing is required to block external air.
Furthermore, a mold is small in mechanical strength such as hardness. As such, when a thermal fuse is being fabricated a spring's force can deform the mold, resulting in a defect. Furthermore, if a completed thermal fuse is stored at high temperature in high humidity the thermosensitive material sublimates, deliquesces and the like, which can affect the product's longevity and also impair its electrical characteristics. Conventional thermosensitive material employing organic chemical, in particular, when it is exposed to high temperature, significantly softens and deforms. It thus diminishes, resulting in a contact dissociating disadvantageously. Accordingly there has been a need for a thermal fuse using thermosensitive material that is less affected in use by its surrounding environment, chronological variation and the like and also have the thermosensitive material free of defect when the fuse is stored in severe atmosphere, exposed to high temperature and high humidity, toxic gas, and the like.